Water treatment systems configured to remove oil and suspended solids from water to be treated, the water containing oil and suspended solids, by using filtration membranes are known. In such water treatment systems, by configuring the systems such that water to be treated is supplied from one end side of filtration membranes in a direction parallel to the membrane surfaces and discharged from the other end side of the filtration membranes, that is, crossflow filtration is performed, clogging of the filtration membranes can be reduced even in the case of treating water to be treated, the water containing large amounts of oil and suspended solids, such as oilfield-produced water.
In addition, by mixing a gas with water to be treated, clogging of filtration membranes can be suppressed by the scrubbing effect of gas bubbles. As a method for mixing a gas with water to be treated, there has been proposed a method using an aspirator (also referred to as an ejector) that produces a negative pressure by means of the Venturi effect due to constriction of a passage of water to be treated and that aspirates a gas into the water to be treated (refer to Japanese Unexamined Patent Application Publication No. 2009-148673).